CKM 2018 M. Rotondo
Marcello RotondoLaboratori Nazionali di Frascati
On behalf of the LHCb collaboration
CKM 201810TH INTERNATIONAL WORKSHOP ON THE CKM UNITARITY TRIANGLE
UNIVERSITÄT HEIDELBERG, SEPTEMBER 17-21, 2018
Semileptonic Λb→Λc(*)μν
Decays
2CKM 2018 M. Rotondo
Why b → c ?● B → Dμν and B → D*μν decays well studied at B-Factories
● A lot of information about B→D**μν and B → Dπ(π)μν also available
● Λb (bdu) have different spin structure and because the (ud) di-quark has j=0,HQET makes clean predictions
● LHCb has the unique capability to study indetail the semileptonic Λb decays
● Only few measurements(Delphi, CDFII) available forsemileptonic Λb
3CKM 2018 M. Rotondo
b → c ● Measure differential spectrum
b
u
W-Vcb
c
u
d d
ΛbΛc
ν
μ-
● Check precise lattice results
● Test HQET predictions in baryons
● First step toward a precise |Vcb| frombaryon decays
● Extract information on function ξB(w) assuming parameterizations based on
phenomenological models or simple expansion around w=1
Detmold, Lehner, Meinel, Phys.Rev.D92,034503(2015)
4CKM 2018 M. Rotondo
b → c: yields and backgrounds● Run1 data: 3fb-1
Very large and clean sample of Λb → Λc μ ν X
Main peaking backgrounds:
- Λb → Λc* μν with Λc* → Λcπ+π- and Λcπ
0π0
Fit on data using Λcπ+π- decay which covers 2/3 of
the Λc* decays
- Λb → Σc++π μν and Σc
0π μν with Σc → Λcπ From data reconstructing Σc
→ Λcπ
Λc(2595)
Λc(2625)
Λc
?(2765) Λc(2880)
Significant yields with excitedstates: opportunity to study them
Measured raw yields
PRD96,112005(2017)
5CKM 2018 M. Rotondo
Reconstruction of the q2
● The knowledge of the Λb momentum Pb isneeded to measure q2=(Pb - Pc)
2
● No constraints from beam energy as at B-Factories
● Hypothesis of just 1-neutrino missing and thewell-measured Λb flight direction gives themomentum with a 2-fold ambiguity, P+ and P-
● Without selection both solutions have samechances to be the correct
● After all selections the solution with smaller Pb momentum is more often the correct one
● Ciezarek et al JHEP02(2017)021
● The q2 resolution can be improved exploiting otherinformation as decay length and angle with respectto the beam line
● Important when angular variables will be considered
● JHEP02(2017)021
6CKM 2018 M. Rotondo
Extraction of the q2 spectrum● Sample of Λb → Λc μ ν X extracted in 14 bins of q2 (take lower pΛb solution)
● Correct for feed-down from peaking backgrounds in each bin
● Correct for selection efficiency
● Distribution unfolded with SVD technique (regularization parameter chosen fromsimulation)
PRD96,112005(2017)Background subtractedΛb → Λc μ ν candidates Unfolded distribution (7 q2 bins)
7CKM 2018 M. Rotondo
b → c: results
Taylor expansion fit● Different parameterizations have good fit quality:data/HQET predictions agree● Knowledge of Λb → Λc form-factors crucial for R(Λc)● A suitable normalization would allow |Vcb| extraction● Open the route to measurements of FF in other B-hadrons
● Comparison with recent lattice calculationshows good agreement
● Support the lattice calculation used in the|Vub|/|Vcb| measurement
● In future further L-QCD calculations would bereally desirable!
Lattice – QCD calculationDetmold at al. PRD92(2015)034503
PRD96,112005(2017)
8CKM 2018 M. Rotondo
Excited states Λc1/2 and Λc
3/2
● Interesting opportunities to study Λb → Λc* μν in particular the copious Λb → Λc(2595) μνand Λb → Λc(2625) μν channels
● Interesting in near future for LFU test with Λb semi-tauonic decays
● Reduced feed-down from higher order excitedstates
● Λc*→Λcπ+π-, di-pion allows a clean
experimental signature
● Theoretical papers on these decays
● Leibovich, Stewart PRD57(1998)5620
● Pervin et al. PRC72(2005)035291
● Gutsche et al. arXiv:1807.11300
● Sensitivity in LHCb to the form factors inthese decays has been investigated in Böer et al. JHEP06(2018)155
Λc(2625)
Λc(2595)
9CKM 2018 M. Rotondo
b → c*: LHCb sensitiviy● Decomposing the Λb → Λc
J μν decay rate in helicity basis
● 6 form factors for 1/2 state
● 8 form factors for 3/2 state
● Up to 1/m corrections can be reduced to two independent Isgur-Wise functions
● Interestingly the same functions describe both states
Coefficients a,b,cdepend on J and Lepton kind
● For unpolarized Λb the differential decay rate is
● Strategy for the sensitivity study in LHCb
● Parametrize the relevant form-factors with a phenomenological model
● Generate and fit toys at different luminosity scaling properly the yields extracted in LHCb
– Considering the resolution on q2 and cosθℓ as in JHEP02(2017)021
● Böer et al. JHEP06(2018)155
10CKM 2018 M. Rotondo
FF parameters sensitivity ● Form-factors parameterized with exponential functions
● Fits with different configurations:
– Separately for Λc1/2 and Λc
3/2
– 1-Dimensional q2
– 2-Dimensional combined q2 and cosθℓ
2D fit Bothresonances
Bothresonances
● Best sensitivity with simultaneous 2Dfit on both resonances: analysisongoing in LHCb
Sensitivity corresponding to the data availableat the end of Run2 ~20K Λc
1/2 and ~50K Λc3/2
● Böer et al. JHEP06(2018)155
11CKM 2018 M. Rotondo
Expectations for R(Λc*)
● From recent calculations from Gutsche et al. arXiv:1807.11300
● The decay into ground state is morefavourable because of the large BF, higherefficiency
● With higher statistics the excited stateswould allow better control of thesystematics due to the peakingbackgrounds
● Böer et al. JHEP06(2018)155
12CKM 2018 M. Rotondo
Outlook
● Properties of semileptonic decays of b-baryons can be studiedin LHCb with high precision
● Great opportunities
● Measurements of CKM parameters, LFU tests● hope to get soon similar/better level of knowledge as in B
meson decays
● Crucial interplay with theorists
● L-QCD is an essential ingredient but it usually requires time● Predictions using other approaches are of course very welcome
● News from baryons in the next months!
13CKM 2018 M. Rotondo
BACKUP
14CKM 2018 M. Rotondo
|Vub| at LHCb● B-baryons provide complementary
informations to B-mesons
● Copious production of Λb
μ-
b
u
W-Vub
u
u
d d
Λb
● Kinematic constraints allow thedetermination of the pΛb (modulo 2-foldambiguity)
● Large background from Λb→Λcμν
● LHCb determines (in the high q2 region) the ratio
Signal
Normalization
● Precise F.F.calculation on L-QCD
– Detmold et al PRD92(2015)034503
Λb→pμν
Λb→Λcμν
q2>15 GeV2
q2>7 GeV2
15CKM 2018 M. Rotondo
b → p signal & |Vub|
Nsig=17687±733
B→π+ℓν
Λb→pμν
Inclusive |V cb|, global fit in KS|Vub|, GGOU
UTfitM.Bona @CKM16
σtot=7%
Including exclusive |Vcb|
Systematics dominated byBF(Λc → pKπ)=(6.46 ± 0.24)%
HFLAV using BESIII-Bellemeasurements
CKM 2018 M. Rotondo
New global picture ?
B→π+ℓν
Λb→pμν
Inclusive |V cb|, global fit in KS|Vub|, HFLAV GGOU
CLNB → D*ℓν
CLNB → Dℓν
Belle BGLB → D*ℓν
Belle BGLB → Dℓν
UTFit
BaBar Ee GGOU
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